Hello Rob,
to prevent the height problematic (e.g. first image has a height with 150 px, the second has 200px and the third has 180px) this Module based on dimensions. That means you define the resolution e.g. 16:9 and the Module will create from the directory images with these resolution. That will grant you the same height of each image.
Please have a look at these link.
www.extro-media.de/en/manual-responsive-fastslider.html
Regards
Caro